Field of the Invention
The invention concerns a method for acquiring magnetic resonance data from a target region of a patient while the target region moves due to respiration, using a single-shot turbo spin echo sequence as a magnetic resonance sequence in a magnetic resonance apparatus, wherein SPAIR fat saturation is used in a data acquisition sequence, by emitting an inversion pulse at an inversion time before data acquisition in the sequence, and wherein multiple repetitions of sequence are respectively triggered by a respiratory signal describing the respiratory cycle upon fulfillment of a recording criterion. The invention additionally concerns a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
In magnetic resonance data acquisitions from regions that are affected by respiration of the patient, for example in the chest region, the movement caused by respiration represents a problem which can result in poorer quality magnetic resonance data if the respective data acquisitions occur at different phases of the respiratory cycle and/or too much movement occurs during the data acquisition. Hence various recording techniques are known to address this problem, most of which are based on triggering by a respiratory signal. For example, the data recording (acquisition) can be triggered when a particular recording criterion indicating a resting phase of respiration in the respiratory signal is fulfilled. For example, such a trigger can be set for a particular expiration value or the like.
However, it is frequently the case with such measurements, especially in the chest region, that fat saturation is also desired in order to suppress fat signal components in the acquired magnetic resonance data. A frequently used recording technique combines single-shot turbo spin echo measurements (single-shot TSE measurements), for example a HASTE sequence (HASTE=Half-Fourier Acquisition Single-Shot Turbo Spin Echo), with SPAIR fat saturation (SPAIR=Spectral Adiabatic Inversion Recovery). In this case, a spectrally selective adiabatic inversion pulse is emitted that inverts the fat spins in the analyzed volume. The fat spins now decay in accordance with the T1 relaxation time, and after a particular characteristic time (inversion time) the longitudinal magnetization is zero, so that at this point an excitation pulse can be emitted. With SPAIR the achievement of a steady state is a prerequisite for uniformly good fat saturation. The inversion time, for which it is necessary to wait after the emission of the inversion pulse before the actual data recording, depends on the repetition time, which results in longer inversion times (TI) in the case of single-shot turbo spin echo sequences, this being the case for the HASTE sequence in particular.
In the case of respiratory triggering, this means that the SPAIR inversion pulse is emitted initially and the actual data recording follows only after a (long) waiting time, the inversion time TI. Since the recording criterion is used to evaluate the respiratory signal such that it can be determined as reliably as possible that a relative rest phase in the respiratory movement will follow, the moment of triggering and thus the moment of emitting the inversion pulse is not randomly displaced within the respiratory cycle, so that with long inversion times there is a risk that during the data recording, in which the single-shot TSE sequence is executed, a stronger respiratory movement is present and thus poorer quality magnetic resonance data are recorded.
Furthermore, the inversion time is ultimately the result of the time needed for the longitudinal magnetization of the fat spins to achieve zero crossover. This, in turn, depends on how high the longitudinal magnetization was before the inversion pulse was emitted. Thus if several sequences or measurement blocks each including an inversion pulse, an inversion time and a magnetic resonance sequence, are performed in successive respiratory cycles, a particular respiratory frequency must be taken as the basis for determining how far the longitudinal magnetization of the fat spins has already progressed, from which the inversion time results. However, if the respiratory frequency changes, the actual previously-calculated inversion time is no longer correct. The effects are in this case not a complete failure in fat saturation, but disruptive fluctuations can occur in the fat intensity.